It has been known that a piece of user equipment (UE) such as a mobile phone changes a base transceiver station (BTS) with which the user equipment is wirelessly communicating, to another base transceiver station, when the user equipment moves. In the description below, changing a base transceiver station with which a piece of user equipment is communicating to another base transceiver station will be referred to as a handover (HO). Further, for ease of explanation, “a piece of user equipment” will be also expressed as “a user equipment” or abbreviated as “a UE”.
When many user equipments gather in the area of the particular base transceiver station and the communication load imposed on a particular base transceiver station is increased, the base transceiver station having the increased communication load may hand over some user equipments being located within the area thereof to another base transceiver station having a small communication load. Hereinafter, another base transceiver station to which a handover is performed will be also expressed as “a handover-destination base transceiver station” or abbreviated as “a HO-destination BTS”, and a base station from which a handover is performed will be also expressed as “a handover-source base transceiver station” or abbreviated as “a HO-source BTS”. In this way, communication loads are leveled between a handover-source base transceiver station and a handover-destination base transceiver station.
In this case, the base transceiver station may select, as a user equipment to be handed over, a user equipment that receives a radio signal from a handover-destination base transceiver station with a large reception power value, from among the multiple user equipments so that a proper wireless communication speed can be ensured after the user equipment is handed over to the handover-destination base transceiver station.
However, in the related art such as Japanese National Publication of International Patent Application No. 2007-525893, selection of a user equipment to be handed over is performed without taking into account differences in reception performance among multiple user equipments.
For example, when multiple user equipments exhibit approximately the same radio signal reception performance, the reception power value of each user equipment reflects the distance between the user equipment and a handover-destination base transceiver station. However, multiple user equipments may include ones having a reception beam formation function by which a radio signal transmitted from a particular direction is selectively received with increased reception sensitivity, and ones not having the reception beam formation function. Accordingly, the multiple user equipments differ from one another in radio signal reception performance.
Further, even user equipments having reception beam formation functions may differ from one another in the performance of the reception beam formation functions. As a result, the user equipments differ from one another in radio signal reception performance. Accordingly, the reception power value of a user equipment may fail to correctly reflect the distance between the user equipment and the handover-destination base transceiver station.
For example, suppose that one of multiple user equipments C and D, which are communicating with base transceiver station A, is handed over to handover-destination base transceiver station B. Assume that the user equipment C is distant from handover-destination base transceiver station B and close to base transceiver station A, and the user equipment C has the reception beam formation function that is oriented to handover-destination base transceiver station B. Also assume that the user equipment D is close to handover-destination base transceiver station B and distant from base transceiver station A, and the user equipment D does not have the reception beam formation function. Since the user equipment C has higher reception performance, the reception power value of a radio signal received by the user equipment C from handover-destination base transceiver station B may be larger than the reception power value of a radio signal received by the user equipment D from handover-destination base transceiver station B. Thus, although user equipment C is more distant from handover-destination base transceiver station B, base transceiver station A may select the user equipment C as a user equipment to be handed over to handover-destination base transceiver station B.
In this case, radio signals transmitted between the user equipment C and handover-destination base transceiver station B may significantly interfere with radio signals transmitted or received by other user equipments belonging to base transceiver station A. For example, base transceiver station A is able to assign wireless resources such as frequencies to user equipments belonging to base transceiver station A so as to reduce interference between the user equipments as much as possible, whereas base transceiver station A is unable to grasp the wireless resources assigned to user equipments belonging to handover-destination base transceiver station B. In addition, there may be many user equipments belonging to base transceiver station A and communicating therewith around user equipment C that is close to base transceiver station A. Further, since user equipment C is distant from handover-destination base transceiver station B, it consumes a large transmission power to transmit a radio signal to handover-destination base transceiver station B. For these reasons, the handover of user equipment C to handover-destination base transceiver station B is considered to significantly increase interference between first radio signals transmitted between user equipment C and handover-destination base transceiver station B, and second radio signals used by the other user equipments belonging to base transceiver station A.
Further, when a user equipment that is close to a handover-destination base transceiver station is simply selected as a candidate user equipment to be handed over, wireless communication may not be favorably performed between the selected user equipment and the handover-destination base transceiver station if the user equipment has poor reception performance.